Radio receiving system



Dec. 30, 1941.

c. w. HANSELL RADIO RECEIVING SYSTEM File'd Nov. 24, 1939 3 Sheets-Sheetl T0 ANTE/YNA n TRANSMISSION L/IVE Fig. 2

UTPUT 7'0 A/EXTSTAGE r0 ANTENNA PULS/NG 0sc/4AT0R OUTPUT z E AMPL/F/ERSSECOND 0575070? AND AUDIO AMPLIFIERS I HETERODYNE ZI I POTEA/T/OMETERINVENTOR CLARENCE W. F4IV$ELL NAM ATTORNEY I BEA T/IVG OSCILLA 70RPULSl/VG OSCILLATOR 12 Dec. 30, 1941. c. w. HANSELL 2,267,732

RADIO RECEIVING SYSTEM Filed Nov; 24, 19:59 5 Sheets-Sheet 2 F/L TER r0ANTENNA I j 010,000.; 400/0 AMPLIFIER POWER I V Y V V V r 5 0 U R C EPULS/A/G OSCILLA r02 #7 70 A uo/b UTILIZATION DEV/CE AUDIO 3/ OUTPUTINVENTOR CLARENCE W HANSELL ATTQRNEY Dec. 30, 1941. I c. w. HANSELL2,267,732

RADIO RECEIVING SYSTEM Filed Nov. 24, 1939 3 Sheets-Sheet 3 r0 ANTENNA44 0 AN ENNA {12E 5 jmnnn I 36 INVENTOR CLARENCE W. HANSELL BY gATTORNEY Patented Dec. 30, 1941 RADIO RECEIVING SYSTEM Clarence W.Hansell, Port Jefferson, N. Y., assignor to Radio Gorporationof America,a corporation of Delaware Application November 24, 1939, Serial No.305,800

17 Claims. 1C1. 250-20) The present invention relates to improvements inradio receivers.

In the reception of very high frequency signals, the input impedance ofthe first vacuum tube is a large factor in loading the input circuit.This loading reduces appreciably the amount of oscillatory energyapplied to the tube and is often responsible for a low ratio of signalto noise, particularly a low ratio of signal to tube noise and thermalagitation noise.

One of the objects of the present invention is to provide a receiverwhich possesses an improved signal to noise ratio, and particularly animproved ratio of signal to noise in the input circuit for the firststage.

Another object of the invention is to obtain greater selectivity in thefirst circuits of a receiver.

A further object is to simplify receivers by eliminating the requirementfor heterodyning to a lower frequency in order to obtain a desireddegree of selectivity.

The foregoing objects and others, which will appear later from a readingof the description of the invention, are achieved in one particular emibodiment by storing a relatively large amount of signal energy in theform of high frequency oscillations in a low power factor resonantcircuit and periodically effectively connecting the storage circuitsuddenly to the signal grid of the first vacuum tube of the receiver, orby rendering the vacuum tube periodically active in a manner toaccomplish an equivalent result. For this purpose, the vacuum tube inputelectron current loading is first effectively removed from its input 9circuit and the input circuit correctly coupled to the source of signaloscillations, usually an antenna or transmission line. In this way amuch larger current and voltage is built up in the low power factorinput circuit than can be obtained by having the vacuum tube effectivelyconnected, or maintained continuously active, in the input circuit.After the large amount of energy has been stored in the low power factorinput circuit, it is quickly used to momentarily control electroncurrents by means of power input to the control grid of the first tubein the receiver, thus providing momentarily a greatly improved signal tonoise ratio. If this is repeated periodically, it should be apparentthat the power available for the input of the tube is momentarily muchhigher than the maximum obtainable continuous or average value. Thereceiver circuits are designed to be responsive only during the periodsof high signal to noise ratio.

In practice, it is proposed to increase the input impedance of the firstvacuum tube during receiver off periods by applying a high negative biasto the control grid, or to a space charge 2 grid between the cathode andthe control grid.

As an illustration of the benefits to be derived from the presentinvention, let us assume that the first vacuum tube of the receiver,under one set of operating conditions at very high frequencies, has aneffective input resistance of 2000 ohms, and that the receiving antennais connected with first circuit of the receiver over a transmission linewhich has its characteristic impedance matched by the input impedance ofth first circuit. If this first circuit is a concentric line resonator,its power factor may be as low as one part in ten thousand. Assume that,when used in the ordinary way, 10% of the received power is to be lostin the first circuit of the receiver (i. e., tuned grid circuit) and 90%applied to the input of the first tube. This would result in a finaloverall frequency selectivity for the input circuits to the first tubeequivalent to a tuned circuit with a power factor of substantially onepart in 500 (taking into account antenna, transmission line, circuit andtube loading). In the assumed case, if we now remove the tube loadingand rematch the antenna line to the circuit we can build up in thecircuit an oscillatory stored energy much larger than before. The powerfactor can then be reduced to only one part in five thousand (takinginto account antenna loading and circuit losses), and with a givenstrength of signal to be received We can build u p ten timesgreateroscillatory energy and V10 times greater voltage and current inthe input circuit. Furthermore, the circuit selectivity is now ten timesas good as before.

If now the bias on the vacuum tube is momentarily reduced so as to throwthe tube input electron loading on the circuit suddenly, it is possibleto deliver momentarily to the vacuum tube ten times as much signal powerwith at least ten times greater signal to noise power ratio. If thevacuum tube is made to be effective repeatedly for short pulses, withrelatively long off periods between them, we thus have a receivingsystem capable of delivering approximately ten times better signal tonoise power ratio than in the original assumed ordinary case. If theintervals between pulses are made substantially equivalent to the timeconstant of the input circuit and are then made of shorter and shorterduration with correspondingly greater and greater momentary circuitloading by the mission line TL is connected across a correct portube,there should be obtained greater and tion of the tuned circuit so thatthe charactergreater signal to noise power ratio. pulse period is 1% ofthe total time and 99% of the time of the ordinary signal pulse is usedto build up or store energy, then the power input to the vacuum tubeduring active periods may be about one hundred times greater than whenthe tube is active continuously, assuming that there is no limit on thepower factor of the input tuned circuit or that thefrequency of pulsesis great enough to neglect power factor. The noise power built up in theinput circuit at, or just before, the beginning of each pulse periodshould be only as high as in the ordinary case, because of increasedcircuit selectivity, and the noise introduced by shot effect, andthermal agitation, upon the first grid, after the tube beistic impedanceof the line TL is matched by the resonant input resistance of thecircuit.

Under these conditions maximum power is delivered to the circuit. Also,the transmission line TL and the resistances R and R across the circuithave equal effect in determining the circuit frequency selectivity,

If now, the 2000 ohm load R is removed from the circuit 5|, 52, R andthe connection of the comes active, should be only 1% as great as in.

the ordinary case. This is the theoretical optimum for the conditionsassumed. In practice, however, all of the theoretically possible gaincannot be realized. In the case just given, in order to make practicaluse of the principles described, there should be one or moreactivepulses for the vacuum tube for each cycle or pulse of usefulmodulation, even though the active pulses are separated from one anotherby one hundred pulse time intervals.

Thus, by storing up the received power in a flywheel circuit, and thenusing it at a high rate for a small percentage of time, we may make thesignal much higher in proportion to tube and circuit noise during theused periods.

Although the principles of the invention are described herein inconnection with the first vacuum tube of a receiver, it. should bedistinctly understood that they are equally applicable at any point inareceiver or signalling system where it is desired to overcome theeffects of an ob-.

jectionable source of noise. For example, it may be desirable toovercome hum or other local noises in communication circuits in one ormore stages following the first stage. The invention findsapplication inreceivers capable of receiving signals from continuous wave telegraphand similar transmitters as well as in receivers capable of receivingspeech waves.

The following is a detailed description of the invention in conjunctionwith the drawings, wherein Figs, 1 to 9, inclusive, show receivercircuits illustrating different embodiments of the invention. Throughoutthe figures of the drawings the same reference numerals are intended todesignate the same or equivalent parts.

Referring more particularly to Fig. 1, there is shown an arrangement ofcircuits for illustrating the principles of the invention. Thesecircuits might be capable of being used at relatively low frequencies.In Fi l I have shown a transmission lineTLfor delivering power to acircuit 5|, E52 tuned to the power frequency. This tuned circuit isconstituted by an inductance coil 5| in parallel relation to a condenser52. We may assume that the reactances of the inductance 5| and thecapacity 52 are each 2 ohms at resonance and that losses in the circuitare equivalent to those which would be produced in a parallel resistanceR of 20,000 ohms connected across the tuned circuit 5|, 52. The usefulload, here indicated by R, is shown connected across the tuned circuit,or removed, by means of a switch or relay S. Let us assume this load Rto be 2000 ohms.

To correspond with the usual case, we may assume that the 2000 ohm loadR is connected across the tuned circuit 5|, 52 and that thetranstransmission line TL to the circuit is readjusted to restore animpedance match between the characteristic impedance of the line TL andthe input impedance of the circuit 5|, 52, R, then the oscillatoryenergy which can be built up in the circuit 5|, 52 by a given availablepower input will be increased'in the ratio of 11 to 1. Therefore, byconnecting the 2000 ohm load R across the circuit momentarily, we candeliver power to the load R at a rate which begins at eleven timeshigher than could be obtained if the load were continuously applied. Themean or average increase in power delivered to the load, for the wholeduration of the load connection, may be say 10 to 1. Therefore, byconnecting the 2000 ohm load R to the circuit 5|, 52 repeatedly, withonly momentary contact and sufiicient time between contacts, we mayobtain an increase of about 10 to 1 in power delivered to the load,during active periods. If the load were a part of a signalling system,and contained a source of noise, we might therefore obtain a 10 to 1increase in signal to noise power ratio during active periods.

In Fig. 2 there is shown an arrangement for employing the principlesdescribed in Fig. l in improving the signal to noise ratio in a veryhigh frequency receiver circuit. This receiving circuit comprises avacuum tube I0 used as the first tube in the receiver, a low powerfactor concentric resonant line input circuit M connected to the controlgrid of tube l0, a transmission line |3 extending from concentric line Mto a receiving antenna, not shown, a pulsing oscillator l2 here shownconventionally in box form, and a pulsing control vacuum tube coupledbetween the oscillator and the first tube. The power received on theantenna is brought to the low power factor tuned circuit l4 over thetransmission line I3, from which energy is fed to the control grid ofthe first tube I0. (In some cases a multi-grid tube may be used insteadof the three electrode tube shown.)

Concentric line I4 should be considered as being typical of any desiredtype of low loss, low power factor tuned circuit which can be used,including resonant circuits made up of concentrated inductance andcapacity as well as resonant cavities in conducting enclosures. In theform illustrated, it is electrically one-quarter of a wave long andconsists of an outer conductor and a coaxial inner conductor bothconnected together at one end by a metallic plate which is connected toground and to the cathode of tube I. This type of resonator is now wellknown in the art and is described in some detail in the article byClarence W, Hansell entitled Resonant lines for frequency control,published in Electrical Engineering, August, 1935, pages 852 to 857, towhich reference is made for a more detailed description.

During most of the time the first receiver tube W has a strong negativedirect current biasing potential on the control grid which effectivelykeeps the space inside the tube clear of free electrons. Under thiscondition the tube input impedance, most of the time, is almost as highas if the cathode were not heated. During these periods of high negativebias the tube loads the low power factor concentric line l4 circuitrelatively little and, by proper impedance matching of circuit M totransmission line I3, a relatively high signal current may be built upin the circuit M if this circuit is tuned to the signal. Also, thefrequency selectivity of the resonant input circuit I4 is relativelyhigh because the only substantial loading on it is its own losses andeffective loading of the antenna line 13. I

By means of a pulsing control tube ll, normal operating bias may beapplied to the first receiver tube Ill momentarily, at frequentintervals. This happens whenever thepulsing control tube I I has itsnegative grid potential swung below cut-off, for the anode circuit, bythe oscillator input [2. A biasing power source, through resistance l5,supplies normal grid bias for tube l0 whenever the anode current of tubeH is cut off, that is, when the tube l l is non-conducting. When ourrentflows in tube II, the potential drop through I resistance I5 isincreased to such an extent that the first tube I0 is made to benon-conducting.

To obtain pulsing circuits, or pulsing oscillators, one skilled in theart may draw upon the radio television art in which pulsing oscillatorsand circuits are used.

During the moments of normal bias, the first receiver tube I0 fills upwith moving electrons and is capable of being used as an amplifier ordetector according to the design of the receiver. Of course, thepresence of the electrons gives the tube a relatively low resistivecomponent of input impedance. However, this low input impedance isthrown as a load upon the input circuit [4 only momentarily instead ofcontinuously as in the,

ordinary receiver.

Due to the momentary character of the loading, the value of loading,while it lasts, is relatively very high and the tube is strongly excitedby the relatively large energy stored in the input circuit Hi. If theactive periods are say of the total time, then power may be taken fromthe input circuit at approximately ten times the rate which could bepermitted in an ordinary receiver and will be at a rate correspondinglyhigher than the rate at which noise energy is generated in the tube.

If, as an example, the receiver is used to receive signals from atransmitter with an ultra short wave carrier frequency of 200,000,000cycles per second, then the pulsing frequency might be on the order of50,000 cycles with 5% on periods. Then each pulse would have the benefitof 4000 cycles of stored energy utilized during a period of 200 cycles.

If the effective power factor of the receiver input circuit is made tobe as low as one part in 20,000, it alone can be relied upon for nearlyall the high frequency selectivity of a radio telephone receiver. Thefirst tube might then be used as a detector and followed only by a lowpass filter to pass 0 to 10,000 cycles and an audio amplifier. Such anarrangement is shown in Fig. 4.

Fig. 3 is a modification of the receiver of Fig. 2 and shows how theprinciples of the invention can be applied to a superheterodynereceiver. In Fig. 3, the first heterodyne detector is supplied withenergy both by the receiving antenna 21 and by the first beatingoscillator 22. The low power factor energy storage circuit 23 is in theoutput of the first detector and. comprises the input circuit feedingthe control grid of the first intermediate frequency amplifier vacuumtube 24, here shown as a screen grid device. The local oscillator 22produces oscillations which are strong enough to eliminate from theoutput of the first detector 20 everything except the beat frequencieswith the oscillator carrier. The pulsing oscillator I2 is here shown asfeeding a push-pull pulsing tube circuit II. This pushpull pulsing tubecircuit allows the first intermediate frequency tube 24 to be activeonly when the pulsing tubes have nearly zero instantaneous grid voltagefrom the pulsing oscillator I2. The output from vacuum tube 24 is, ifdesired, fed to one or more suitable intermediate frequency amplifierstages and to the second detector and audio amplifier stages here shownlabeled in box form as 25, from which the signal can be heard in aloudspeaker or headphones. Any power at the pulsing frequency, or itsharmonics, which does not balance out in the pulsing tube circuit II, iseliminated by frequency selectivity of the later circuits. Thearrangement of Fig. 3 provides greater selectivity in the intermediatefrequency circuits.

Fig. 5 illustrates a receiver wherein the first stage, supplied withenergy from the antenna, is a detector having a push-pull input. In thisfigure the antenna feeds a low power factor resonant line system 26,here shown as a half wavelength coaxial line, via a pair of coaxialtransmission lines 27. The first stage comprises a pair of vacuum tubes28 whose grids are coupled to the resonant system 26 on opposite sidesof the voltage nodal point. It should be noted that both the feederlines 21 as well as the grids of the tubes 23 are symmetrically coupledto the resonant line 26 on opposite sides of its center. The outputsfrom the vacuum tubes 28 are in parallel and feed the audio frequencyenergy to a suitable audio frequency amplifier and/or loudspeaker. Inthis arrangement, the lead 29 in the grid circuits of the vacuum tubes28 extends to a pulsing tube and pulsing oscillator arrangement of atype similar to that shown in Fig. 1 for supplying periodic bias pulsesto render the tubes 28 conductive and non-conductive.

Fig. 6 shows a receiver arrangement which is a modification of thecircuit of Fig. 5. In Fig. 6 the push-pull vacuum tubes 30 constitutingthe first stage function as amplifier tubes whose outputs feed adetector 3|, in turn providing audio frequency output signals. Onefeature of this circuit is that the pulses in anode direct current oftubes 30, at the pulsing frequency rate, are eliminated by bothfrequency selectivity and balancing.

If desired, we can use an extremely low power factor circuit in theoutput of the first receiver tube or stage of Figs. 2, 3, 4, 5 and 6which is capable of smoothing out pulse modulation of the received highfrequency energy but not the useful modulation, in which case we wouldhave aradio frequency amplifier for amplifying the incoming modulatedcarrier, thus giving a much lower inherent receiver noise level.

Fig. 7 shows a receiver arrangement particularly suitable for lowfrequency operation. The lower power factor tuned circuit comprising aninductance coil and a parallel condenser, the combination of which ishere labeled 32, is excited by energy from feeder 33 extending to apower source, which may be an antenna, not

shown. By means of a vibrator 34, the terminals of the tuned circuit 32are periodically shunted by a condenser 35 which is of sufiicient valueto change the normal resonant period of 32 from a radio frequency to anaudible frequency which can be heard in headphones 36. This shunting ofthe tuned circuit 32 causes the oscillations therein to take place as adamped audio frequency wave which is audible in the headphones. In thecircuit of this figure, as well as in the circuits of the previouslydescribed figures, oscillatory energy of relatively large value is builtup in the unloaded low power factor circuit. This energy isintermittently utilized, by momentarily loading the circuit, in thiscase by means of vibrator 34, and the headphones 36.

Fig. 8 shows an arrangement also suitable for the reception of lowfrequencies in which coils 3'! are saturated iron core inductancesarranged in series to each other but in parallel to a condenser 38 toform a tuned circuit for radio frequency energy. By means of a rotatingcommutator 39 periodically supplying direct current to coils 40 coupledto coils 3'1, the saturating current for inductances 31 is interrupted,thus changing the tuning of the circuit 31, 38 to an audible frequency.Connected across the low power factor tuned circuit is a pair ofheadphones M in series with a radio frequency choke coil 32. Condenser33 is a radio frequency bypass for the headphones. It will be apparentthat when coils 4b are properly poled then the radio frequency inducedtherein will balance out. In Fig. 8 the incoming signals received on theantenna are applied to the tuned circuit 31, 38 by means of transmissionline 44 and coils 45, the latter being coupled to coils 31.

Fig. 9 is an alternative arrangement which can be used in place of thecircuits of Figs. 7 and 8. In this figure, the low power factor tunedcircuit 45 in which the oscillation energy is stored has an inductance41 placed in series with the radio frequency inductance of the tunedcircuit. This inductance 41 is short circuited by vibrator 34, whileradio frequency energy is being stored in tuned circuit 46, and thenthrown into the circuit of 46 by the opening of the contacts of thevibrator to bring about the frequency conversion to an audible frequencywhich can be heard in headphones 36.

It should be noted that the input currents and the pulses of usefulcurrent passed on by the pulsing tubes of Figs. 2, 3, 6 may be eitheramplitude modulated, frequency modulated or phase modulated by a usefulsignal modulation, and that the final demodulator should be chosenaccording to the type of modulation applied to the input currents at thetransmitter.

What is claimed is:

1. A radio receiver having a vacuum tube, a low power factor radiofrequency tuned circuit connected to the input of said vacuum tube, andmeans for periodically increasing appreciably the input impedance ofsaid vacuum tube, for a relatively large percentage of the time comparedto the time between said periods and at least once for each pulse ofuseful modulation received on said receiver whereby the loading of saidvacuum tube on said tuned circuit is materially reduced during saidperiods of increased input impedance.

2. In a radio receiver having a vacuum tube and a low power factor tunedcircuit associated with the input of said vacuum tube, the method ofoperation which comprises receiving energy in said receiver, effectivelydissociating said vacuum tube from said tuned circuit, storing saidenergy in said tuned circuit, and then periodically operativelyassociating said tuned circuit suddenly with said input for relativelyvery short periods of time compared to the time between said periodsduring which said input is effectively dissociated from said inputcircuit.

3. In a radio receiver having a vacuum tube, a low power factor tunedcircuit associated with the input of said vacuum tube, and an antennacoupled to said tuned circuit, the method of operation which includesthe step of periodically rendering said vacuum tube inactive for aperiod of time much longer than the time during which said tube isactive and at least once for each pulse of useful modulation.

4. In a radio receiver having a vacuum tube and a low power factor tunedcircuit associated with the input of said vacuum tube, the method ofoperation which comprises receiving energy in said receiver, effectivelydissociating said vacuum tube from said tuned circuit, storing saidenergy in said tuned circuit, and then periodically operativelyassociating said tuned circuit suddenly with said input for a relativelyvery short period of time and at least once for each pulse of usefulmodulation received on said receiver.

5. In a radio receiving system, an antenna, a low power factor radiofrequency tuned circuit coupled to said antenna, a utilization circuit,and means for periodically operatively associating said utilizationcircuit, suddenly, with said tuned circuit for a relatively very shortperiod of time compared to the time during which said utilizationcircuit is disassociated from said tuned circuit.

6. In a radio receiving system, a source of signals, a low power factorradio frequency tuned circuit coupled to said source, a vacuum tubehaving an input electrode connected to said tuned circuit, and means forperiodically rendering said vacuum tube inactive for a period of timemuch longer than the time during which said tube is active, whereby theloading of said vacuum tube on said tuned circuit is effectively removedtherefrom during said inactive periods.

'7. In a radio receiving system, a source of signals, a low power factorradio frequency tuned circuit coupled to said source, a vacuum tubehaving its control grid connected to said tuned circuit, means forapplying bias to said grid, and a circuit including an oscillator forperiodically increasing the bias on said grid to render said vacuum tubeinactive and for suddenly decreasing the said bias on said grid torender said tube active, the inactive periods of said tube beingappreciably longer than the active periods.

8. A system in accordance with claim 7, characterized in this that saidsystem includes another vacuum tube whose input is coupled to saidoscclillator and whose output is in circuit with said gri 9. In a radioreceiving system, a source of signals, a low power factor radiofrequency tuned circuit coupled to said source, a vacuum tube having aninput electrode connected to said tuned circuit, means for periodicallyrendering said vacuum tube inactive for a period of time much longerthan the time during which said tube is active, whereby the loading ofsaid vacuum tube on said tuned circuit is effectively removed therefromduring said inactive periods, a filter coupled to the output of saidvacuum tube,

and an audio frequency utilization circuit cou pled to said filter.

10. An ultra high frequency radio receiving system comprising anantenna, a low power factor tuned circuit coupled to said antenna, adetector and amplifier vacuum tuve having its input circuit connected tosaid tuned circuit, a filter circuit coupled to the output of said tube,an audio frequency utilization device coupled to said filter, and meansfor effectively periodically disassociating the input of said tube fromsaid tuned circuit and for suddenly operatively associating said inputwith said tuned circuit, the periods during which said input circuit isdisassociated from said tuned circuit being longer than the periodsduring which said input circuit is operatively. associated with saidtuned circuit.

11. In a heterodyne radio receiving system, an antenna, a vacuum tubeheterodyne detector coupled to said antenna, a local oscillator alsocoupled to said detector to beat with the waves received on said antennafor producing an intermediate frequency in the output of said detector,a low power factor oscillatory circuit in the output of said detector,an intermediate frequency amplifier vacuum tube having an inputelectrode connected to said low power factor circuit, a

utilization circuit including a second detector coupled to saidamplifier, and means for periodically rendering said amplifier .vacuiuntube inactive, whereby the loading of said amplifier on said low powerfactor circuit is effectively removed therefrom during said inactiveperiods, the periods during which said amplifier is inactive being atleast ten times longer than the periods during which it is active,whereby the value of the energy stored in said low power factor circuitduring the inactive periods which is avail able for said amplifier ismuch higher than the steady state value obtainable if the amplifier werecontinuously active.

12. In a heterodyne radio receiving system, an antenna, a vacuum tubeheterodyne detector coupled to said antenna, a local oscillator alsocoupled to said detector to beat with the waves received on said antennafor producing an intermediate frequency in the output of said d tector,a low power factor oscillatory circuit in the output of said detector,an intermediate frequency amplifier vacuum tube having a grid andcathode connected to said low power factor oscillatory circuit, animpedance in circuit with said grid for applying a negative bias to saidgrid relative to said cathode, a vacuum tube having an output electrodein series with said impedance relative to a source of polarizingpotential, and a pulse oscillator coupled to the input of said lastvacuum tube for periodically causing said last vacuum tube to becomeinactive at the frequency of said pulses, whereby the negative bias tothe grid of said amplifier is increased solely during the active periodsof said last vacuum tube to such an extent that said amplifier is madeto be non-conductive during said active periods, the conductive periodsof said amplifier being very short compared to the nonconductive periodsthereof and at least one for each cycle or pulse of useful modulation.

13. In a radio receiving system, an antenna, a low power factor radiofrequency tuned circuit coupled to said antenna, a utilization circuitalso coupled to said tuned circuit, and means for periodicallyeffectively disassociating said utilization circuit from said tunedcircuit for a period of time at least twenty times longer than the timeduring which said utilization circuitis operatively associated with saidtuned circuit.

14. Means for increasing the ratio of magnitudes of desired currents toundesired currents during periods of utilization of thedesired currents,comprising means for accumulating and storing energy of the desiredcurrents, and means for periodically utilizing the stored energy forvery short time periods compared to the time during which said energy isstored, the periodicity of utilization being at least once for eachcycle or pulse of useful modulation.

15. In combination, a non-oscillating amplifier tube, a source ofalternating current signal energy coupled to the input of said tube, andmeans for periodically effectively disassociating said amplifier tubefrom said source for a period of time at least ten times longer than thetime during which said tube is operatively associated with said source,said amplifier being operatively associated with said source at leastonce for each cycle or pulse of useful modulation.

16. In a high frequency system, a source of high frequency signalenergy, a utilization circuit coupled to said source, and means forperiodically effectively disassociating said utilization circuit fromsaid source for a period of time at least ten times longer than the timeduring which said utilization circuit is operatively associated withsaid source, said utilization cir'cuit being operatively associated withsaid source at least once for each cycle or pulse of useful modulation.

17. In a high frequency signal system having a tuned circuit coupled tothe input of a vacuum tube, the method of increasing the ratio ofmagnitudes of desired currents to undesired currents during periods ofutilization of the desired currents which includes the step ofperiodically rendering said vacuum tube inactive for a period of timemuch longer than the time during which said tube is active and at leastonce for each pulse of useful modulation.

CLARENCE W. HANSELL.

